Nacreous material from titanium dioxide



United States Patent Ofihce DIUXIBE Harold A. a, crest Hills, N.Y., 2"nor to Francis Earle Labor o es, Inc, Peekslsill, Nib, a corporation oftNew i No Drawing. F

d Feb. 27, 1959, No. 795362 s. l lo-448) This invention relates to theutilization of platelets r lamellae of titanium dioxide or hydratedtitanium dioxide in nacreous compositions.

Nacreous preparations are used to impart a pearly or mother-of-pearlappearance to various objects. For example, simulated pearls aremanufactured by coating glass or plastic beads with a suspension ofnacreous pigment in an approp ate vehicle, suc. as cellulose nitratelacquer. Simulated mother-of-pearl knife handles and fountain penbarrels can be made incorporating nacreous pigment in a plastic, such ascellulose acetate, which is then molded into the appropriate shape.Pearl sheets are made by casting polymerizable resins, such as acrylicor polyester resins, in which nacreous pigments are suspended; thesheets can be cut into disks for the manufacture of simulated pearlbuttons.

A Widely used nacreous pigment is Pearl Essence, which is a suspensionof the plate-like guanine crystals derived from certain fish scales.Other nacreous pigments consist of suspensions in suitable vehicles ofcrystals of lead hydrogen phosphate or basic lead carbonate.

These materials all have certain physical characteristics in common:They consist of transparent or translucent plate-like crystals, twomicrons or less in thickness, which have high indices of refraction, andwhich, to be of practical use, are relatively insoluble in Water and inthe organic liquid compositions utilized in plastics work. They alsoshould have melting or decomposition points which are high in comparisonwith the temperature encountered in plastics processing.

When such crystals are incorporated in a transparent or translucent baseand oriented into parallel layers, light is reflected simultaneously ina given direction from a multitude of surfaces, producing the type ofluster which is characterized as pearly. The orientation is madepossible by the plate-like character of the crystals. Inasmuch as theintensity of the reflectance at a boundary between two transparentsubstances depends on the difference in index of refraction, it isnecessary that the index of refraction of the crystal deviate from thatof the medium. Generally, a suitable nacreous effect begins to appearwhen the index of refraction of the medium and that of the crystaldilfer by approximately 0.2 and improves as the difference increases.

In practice the commonly used resins and plastics have indices ofrefraction close to the ran e 1.50 to 1.60, and the nacreous crystalshave indices above 1.70. The indiccs for lead hydrogen phosphatecrystals, for example, are 1.86, 1.83 and 1.8l, while those for basiclead carbonate crystals are 2.09 and 1.94.

in accordance with the present invention, it has been found thattitanium dioxide platelets may be made to give a nacreous effect andthat such a nacreous material will have many advantages over theconventional crystalline nacreous products, having lower solubility inmost solvents and chemicals, high stability at elevated temperatures,and less sensitivity to methods of handling.

Accordingly, it is an object of the present invention to provide anacreous material of titanium dioxide having desirable physical andchemical properties, not heretofore attainable in prior artcompositions.

Another object of this invention is to provide a nacreous coating orother nacreous composition utilizing titanium dioxide platelets.

Another object is to provide a resinous body or other article ofmanufacture having incorporated therewith a nacreous compositionutilizing titanium dioxide platelets.

Still another object of the present invention is to provide a method forpreparing a nacreous composition of titanium dioxide platelets.

These and other objects of the present invention will become moreapparent when taken in connection with the following description.

Although the conventional nacreous substances describ d above arecrystalline, the nacreous efiect can be obtained without the use ofcrystalline materials, so long as the individual particles aresufliciently thin platelets with a suitable index of refraction. Whenlamellae of primarily amorphous titanium dioxide of dimensions similarto those of the nacreous crystals hereinbefore referred to were preparedand incorporated in a vehicle, these amorphous platelets had a pearlyluster like that or" the known nacreous pigments. The usual pearlyeffects were obtained both by coating surfaces with these platelets andalso by incorporating the platelets in transparent plastic compositions.

A convenient method for forming the platelets involves first depositingan extremely thin film of tetraalkyl titanate which is then permitted tohydrolyze to titanium dioxide by the action of water vapor in theatmosphere. The film thickness should be two microns or less. Thesedimensions include films which exhibit an iridescent effect, i.e. showinterference colors. The film is then stripped from the surface on whichit was deposited and is converted to platelets, this process beingeffected conveniently by suspending the film in Water or other liquid,Where it can be broken down by violet agitation or by grinding in anysuitable device, such as a ball mill.

Among the organic esters of tetravalent titanium which are suitablesources of "the titanium dioxide film are tetraethyl titanate,tetraisopropyl titanate, tetrabutyl titanate, tetra-Z-ethylhexyltitanate tetraoctyl tene -e. Generally lower alkyl esters containing upto about 12 car bon atoms may be used, with those con ining 2 6 carbonatoms being preferred. In the presence of moisture, these estershydrolyze so as to produce an alcohol and a titanium dioxide, inaccordance with the following reaction:

The titanium dioxide is amorphous, and may contain some water ofhydration. Nevertheless, its index of refraction is greater than 1.9.

The lower members of the series, such as the tetraethyl andtetraisopropyl titanates, hydrolyze fairly rapidly, while higher membershydrolyze more slowly. Thus a choice of esters, or combinations of twoor more, are possible for particular applications to control the rate offormation of the film. The compounds with lower molecular weight, suchas the tetraisopropyl and tetrabutyl esters, are convenient when it isdesired to perform the hydrolysis at room temperature.

In utilizing the hydrolysis of the titanium ester for the production oftitanium dioxide film in accordance with the coating solution isdetermined by the thickness of titanium dioxide film which is desired.The concentration generally falls within the range of l to 30%, and mostconveniently, 3 to 5% tetraalkyl titanate. It is often found preferableto use a concentration in the lower portion of'the range and to applymultiple coats, rather than to attempt to achieve the desired filmthickness with a single coating. The building of the film in a moregradual manner generally assists in attaining a transparent rather thana whitish, more opaque film.

The nature of the solvent is largely determined by the properties of thesurface to be coated. it is generally desirable to choose a solventwhich has a strong tendency to wet the surface in question uniformly,but which does not attack or dissolve the surface to be coated, as wouldbe the case in certain plastic-solvent or lacquer-solvent combinations.Uniform coating can often be achieved by the addition of surface activeagents, or by the use of a higher ester, such as the tetrastearyltitanate, which has some wetting activity of its own.

The volatility of the solvent or solvent mixture must also beconsidered. if the solvent has too high a rate of evaporation, thecoated surface is rapidly cooled. The atmospheric water vapor whichcondenses at the cool surface may cause the titanium dioxide film tobecome opaque, or blush. If the volatility is too low, drying requiresan inconveniently long time. If the volatility of the compound formed byhydrolysis of the titanium compound is relatively low, a somewhatelevated temperature in the drying may be desirable. To avoid theappearance of chalkiness in the film because of high relative humidity,especially where ambient temperature conditions are employed, therelative humidity should be below 60% and preferably about 50% or less.On the other hand, since the reaction depends on hydrolysis and watervapor from the atmosphere is required, the relative humidity should notbe too low; relative humidities below 20%, for example, would entail aninordinately long time period for film formation. Thus relativehumidities between 20 and 50% are quite adequate, especially in thepreferred ambient temperature range of approximately C. to 35 C.

Hydrolysis takes place i'apidly, and the films are formed andhydrolyzed, using the solvents which will be described in the examplesbelow, within a period of 2 to 20 minutes. Thus, it is not necessary touse forced air or elevated temperatures, although the process can bemade even more rapid by the use of higher temperature up to about 120 C.with the appropriate relative humidity.

Under some conditions the clear film of amorphous titanium dioxidecracks and crazes when hydrolysis is complete, and is thus easilyseparated from the base material simply by scraping off. With thinnerfilms, the use of a lacquer coating is particularly convenient: if thehydrated titanium dioxide film is formed on a thin lacquer coat, theentire system can easily be removed, either by soaking in water andstripping away from the permanent base material, or by dissolving thelacquer film in an appropriate solvent. This method is particularlyconvenient for large scale production, where an endless belt is coatedfirst with a suitable lacquer and then with the tetraalkyl titanatesolution. It is then sent through a suitable drying and hydrolyzingregion and is stripped by one of the methods described above, at whichpoint the belt is returned to its starting position in the cycle.

When this lacquer coating method is used, it is necessary to choose alacquer material which is compatible with the end use for which theplatelets are intended. If the nacreous titanium dioxide is to be usedin coating simulated pearls, for example, nitrocellulose is a suitablecoating material, since nacreous pigments are commonly applied tosimulated pearls from nitrocellulose lacquers. On the other hand, if thetitanium dioxide platelets are to be incorporated in a casein plastics,methyl cellulose makes a sutiable substrate material.

As was indicated above, the film is conveniently broken into plateletsof the desired dimensions by being suspended in a liquid which is thenagitated violently or put through a suitable fragmenting device. Samplesare withdrawn from the suspension at frequent intervals and examined bymeans of the microscope to determine the size of the fracturedparticles. The process is stopped when the titanium dioxide film hasbeen reduced to platelets whose long dimension falls preferably between2 and microns. It is further desired that the ratio of length tothickness be at least 4. Thus, particles of 2 microns length aresuitable if they are 0.5 micron or less in thickness, but platelets 2microns thick should be at least 8 microns long. Although lengths of 2to 100 microns produce optimum luster, lamellae which are even smallerthan 2 microns inlength are suitable if the particles are thin enough,the main criterion being that the crystals remain platelets, which, asindicated above, may be considered to be the case if the ratio of lengthto thickness is four or greater. If during the fragmenting process somesmall particles with a lower ratio of length to thickness are produced,they dilute but of course do not destroy the nacreous effect of the moredesirable platelets.

Platelets longer than 100 microns also produce pearly luster, but areless desirable for many purposes, because individual particles can bedetected by the eye, giving a discontinuous optical effect.

The resulting suspension of titanium dioxide platelets has the typicallysilky appearance of nacreous suspensions. It is heterogeneous withrespect to platelet area and platelet thickness. If desired, thelamellae can be fractionated into more uniform size ranges by settlingand decantation or by centrifuging. The thinner particles which are themost lustrous per unit weight of titanium dioxide since they present thegreatest number of reflecting surfaces, are very well suited to anapplication like the surface coating of simulated pearls, while thesomewhat thicker platelets are preferable in plastic molding operationswhere the greater rigidity of the particles prevents their beingfractured further by agitation in the viscous plastic.

The hydrated, amorphous titanium dioxide platelets have many propertieswhich make them ideal for utilization in nacreous compositions. Theyhave an index of refraction of at least 1.9,. are relatively insolublein most chemicals, and have high temperature stabilities so that theycan be incorporated in plastic molding operations without degradation.Moreover, they are chemically inert and, therefore, unreactive with thevarious media in which they would be suspended. It is possible furtherto improve the quality of the platelets by heating them to a hightemperature, preferably in the range 300 C. to 800 C. During thisoperation the relatively amorphous titanium dioxide platelets becomemore crystalline without losing their gross morphological structure. Asa result, the particles remain in the form of platelets of the desireddimensions, but are increased in refractive index to the range 2.4 to2.9. This increase in refractive index pro.-

duces an increase in light reflectivity.

Another consequence of this heat treatment is that any residual organicmaterial from the hydrolysis reac tion or from a lacquer film substrateis oxidized and volatilized with the result that the platelets then arecompatible with any medium-in spite of the particular lacquer substanceon which they may have originally been deposited. In spite of theadvantages of the heat treatment, however, this additional operation isby no means essential to the utility of the invention.

The nature of the invention is most readily described by means of anumber of examples, which follows:

Example I A 20% solution of tetrabutyl titanate in benzene was prepared.A thoroughly cleaned glass plate measuring 3 feet by 4 feet wasvertically immersed into the solution at room temperature in anatmosphere with a relative humidity of between 40 and 50%. The plate wasslowly withdrawn from the solution and permitted to hang above the diptank for ten minutes. The dipping process was repeated twice. Each timea hard film formed on the surfaces of the glass plate. However, by thethird coat the films cracked and fiaked, and could easily be scrapedfrom the glass with a spatula. The films, approximately 0.5 micron inthickness were taken up in approximately 100 ml. of butyl acetate, thesuspension then being violently stirred until the platelets had anaverage long dimension of 50 microns. The platelets were permitted tosettle out of the butyl acetate, which was decanted for further use,leaving a paste consisting of 25 titanium dioxide in butyl acetate. Tothis paste was added 24 times its weight of a lacquer consisting of 8.0%cellulose nitrate (-20 seconds) in butyl acetate. The resulting pearllacquer was then used for coating alabaster glass beads by dipping, thusmaking simulated pearl beads. in the above example, it is to be notedthat the platelet comprises by weight of the paste composition. Actuallythis composition may vary in accordance with the convenience of theuser, so that the desired ultimate platelet concentration in the finalcoating composition is in the range of about 0.25% to 5.0%.

Example I! Titanium dioxide film approximately 200 millirnicrons inthickness was prepared by immersing a glass plate 3 feet by 4 feet in asolution containing 10% etraisopropyl titanate and 1% Aerosol GT inethylene glycol monomethyl ether under the atmospheric conditionsdescribed in Example 1. Two coats were required, 10 minutes drying beingallowed in each case. The dried plate was allowed to soak in water toloosen the film, which was then scraped off and washed into a smallvolume of Water. The plates were fragmented to an average long dimensionof microns by the procedure of Example I and were then collected bydecantation and dried. The titanium dioxide platelets (1.0 part) weresuspended in 100 parts of polyester castin resin, and the mixture wascast to produce a pearl sheet suitable for cutting into polyester pearlbuttons.

Example 111 A clean rectangular glass plate 3 feetx4 feet was immersedin a dilute lacquer solution consisting of 2% of /2 second cellulosenitrate in butyl acetate. The glass plate was as slowly withdrawn andthe lacquer film permitted to dry, after which the coated plate wascoated twice with a 10% solution of tetrabutyl titanate in mineralspirits. After drying and hydrolyzing as in Example I, the plate wasimmersed in water for about one hour, after which the lacquer filmcontaining the titanium dioxide film was easily stripped from the plate.The collected nitrocellulose film was rinsed with ethyl alcohol toremove water, and was then dissolved in a small amount of butyl acetate.The titanium dioxide film, approximately 100 millimicrons thick, wasfragmented to an average particle size of 50 microns. The titaniumdioxide platelets Were then collected by centritugation and incorporatedin nitrocellulose dipping lacquer for application to simulated pearlbeads as in Example I.

nxnmple IV A glass plate as in Example I was dipped into a 30% solutionof tetra-Z-ethylhexyl titanate in ethylene glycol monoethyl ether. Theplate was dried with forced air at an average temperature of 150 F. anda relative humidity of 25%. The dried film was thick enough to flakeafter a single operation because of the relatively high viscosity of thedipping solution, and was Washed down into a small quantity of water forfragmentation to platelets approximately 70 microns in length. The driedplatelets (0.5 part) were then incorporated in 100 parts of methyl '43methacrylate molding powder for injection molding to make methylmethacrylate pearl articles.

Example V A glass plate was first coated with a solution of 1% gelatinand 0.2% Aerosol OT in water. it was then immersed in a 15% solution oftetraisopropyl titanate in isopropanol, a single coating giving atitanium dioxide film averaging 50 millirnicrons in thickness. Afterdrying under the conditions of Example I, the plate was scraped down ina stream of water and the film was fragmented to platelets having anaverage diameter of about 40 microns. The platelets of hydrated,amorphous titanium dioxide were collected by centrifugation, and heatedto 700 C. for one hour. The resulting platelets had an increased indexof refraction, although they retained the general dimensions of theinitial unheated platelets. They were incorporated in nitrocellulosequest, as in Example I, for the manufacture of simulated pearls.

Example V] A clean rectangular glass plate 3 feet by 4 feet wasvertically immersed in a lacquer solution consis g of 10% by weight ofpolymethyl methacrylate in methyl isobutyl ketone. The glass plate wasslowly Withdrawn and the lacquer film allowed to dry, after which thecoated plate was coated three times with a 15% solution of tetrabutyltitanate in heptane. Each time the plate is withdrawn from the solutionand allowed to dry for 10 minutes. The hydrolyzed titanium dioxide filmis about 0.75 micron thick. The plate is then immersed in water forseveral hours, and then the lacquer film, with its titanium dioxideovercoat, is stripped from plate. The acrylic lacquer film was rinsedwith ethyl alcohol to remove water, and then the film was dissolved inabout grams of methyl ethyl ketone. Aft-er settling, the supernatantliquid was decanted, the settled platelets were stirred vigorously toform platelets less than 100 microns long, and then the platelets weredispersed in 300 grams or" a methyl methaorylate casting syrup, thecrystal concentration being 0.50%. The mixture was then cast using 1.50grams of acetyl per catalyst to make a pearl polymethyl methacrylate sl"suitable for cutting into rnethacrylate pearl buttons.

In the foregoing, the present invention has been described only inconnection with prefer ed embodiments thereof. Many variations andmodifications of the principles of my invention within the scope of thedescription herein are obvious. Accordingly, the invention is to belimited not by the specific disclosure herein, but only by the appendingclaims.

I claim:

1. A nacreous composition, having ing substance therein, ti nium dioxideplatelets suspended in a light transmitting medium, each of saidtitanium dioxide platelets having a thickness not greater than about 100millimicrons, an average ratio or" length to thickness of at least fourto one an index of refraction at least 0.2 greater than that or" saidsuspending medium.

2. The nacreous composition of claim 1, in which said platelets havelengths between 2 and 100 microns.

3. The nacreous composition of claim 1, in which the titanium dioxideplatelets have an index of refraction of at least 1.9 and are suspendedin a film-forming lacquer.

4. A nacreous composition, having as a nacre-producing substancetherein, titanium dioxide platelets suspended in a light transmittingorganic resinous material, each of said titanium dioxide plateletshaving a thickness of no more than about 100 millirnicrons, an averageratio of length to thickness of at least four to one, a

as a nacre-producan index of refraction of at least 1.9, said index ofrefraction being at least 0.2 greater than that of the plastic.

6. An article of manufacture having thereon a nacieous coating, whichhas suspended therein as a naore-producing substance, titanium dioxideplatelets having a thick-- ness of no more than about 100 milllmicronsand an in dex of refraction of at least 1.9, said index of refractionbeing at least 0.2 greater than that of the coating medium.

7. The composition or" claim 1 wherein the nacre-producing substance isin the form of crystalline titanium dioxide having an index ofrefraction of between about 2.4 and 2.9.

8. The article of claim 5 in which the plastic material is from thegroup consisting of acrylic resins, polyester resins and casein resins.

9. The method of preparing nacre-producing titanium dioxide plateletswhich comprises depositing a Smooth substrate film, which is soluble ina solvent which does not dissolve said titanium dioxide platelets, on aclean surface, and, over said substrate film, a film of a solution of alower titanium ester having alkyl groups containing up to 12 carbonatoms, removing the solvent from said solution, and hydrolyzing saidfilm in an atmosphere having a relative humidity of between about 20percent and 60 percent thereby forming a film having a thickness notgreater than 0.75 micron, removing the substrate and hydrolyzed filmfrom said surface, separating the substrate from the hydrolyzed film,and then fracturing said hydrolyzed film, thereby forming anacreproducing platelets.

10. The method of preparing nacre-producing titanium dioxide plateletswhich comprises depositing a mooth substrate film, which is soluble in asolvent which does not dissolve said nacre-producing titanium dioxideplatelets, on a clean surface, and, over said substrate film, a thinfilm not thicker than 0.75 micron of an organic tetra-alkyl titanatefrom the group consisting of tetraethyl, tetraisopropyl and tetrabutyltitanate, hydrolvzing said film to TiO in an atmosphere having arelative humidity between about 20 percent and 60 percent at ambienttemperature, removing the substrate and hydrolyzed film from thesurface, separating the substrate from the hydrolyzed film, and thenfracturing the hydrolyzed film by agitation in a liquid medium which isa non-solvent for the film thereby forming the nacreproducing hydratedTiO platelets.

11. The method of claim in which the hydrolysis is performed in anatmosphere maintained at a temperature between about 10 C. and 35 C. andhaving a relative humidity between about 20 percent and percent.

12. The method of preparing nacre producing titanium dioxide plateletswhich comprises depositing a smooth substrate lacquer film, which issoluble in a solvent which does not dissolve said nacre-producingtitanium dioxide platelets, on a clean surface, then depositing a thinfilm of a tetraalkyl titanate containing up to 12 carbon atoms from asolution of said tetra-alkyl titanate over said substrate lacquer film,hydrolyzing said film to TiO in an atmosphere having a relative humiditybetween about 20 percent and percent and maintained between about 10 C.and C., removing the substrate and the hydrolyzed film from the surface,separating the substrate film from the hydrolyzed film, and thenfracturing the hydrolyzed film by agitation in a liquid medium which isa non-solvent for the film into macro-producing platelets of a thicknessnot greater than 0.75 micron and an average ratio of length to thicknessof at least four.

13. The method of claim 12 including the further step of increasing therefractive index of the nacre-producing TiO platelets by heating thesame to a temperature in the range of about 300 C. to 800 C.

14. The method of preparing haste-producing titanium dioxide plateletswhich comprises depositing a thin film of a nitrocellulose lacquer on aclean surface, then depositing athin film of a tetra-alkyl titanatehavingalkyl groups containing up to 12 carbon atoms oversaid lacquerfilm, hyd rolyzing said film to TiO in an atmosphere containing watervapor and maintained between about 10 C. and 35 C. and having a relativehumidity between about 20 percent and 50 percent thereby forming ahydrolyzed film having a thickness not greater than 0.75 micron,immersing the coated, surface in water, stripping the lacquer film andthe hydrolyzed film from the surface, dissolving the nitrocellulose soas to separate it from the hydrolyzed film, fracturing the hydrolyzedfilm by agitation in a liquid medium which is a non-solvent for the filminto nacre-producing platelets of a thickness not greater than 0.75micron and an average ratio of length to thickness of at least four, andthen incorporating said platelets in a nitrocellulose dipping lacquer.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES Sidlow: Chemical'Products, June 1953, pages 215- 219.

9. THE METHOD OF PREPARING NACRE-PRODUCING TITANIUM DIOXIDE PLATELETSWHICH COMPRISES DEPOSITING A SMOOTH SUBSTRATE FILM, WHICH IS SOLUBLE INA SOLVENT WHICH DOES NOT DISSOLVE SAID TITANIUM DIOXIDE PLATELETS, ON ACLEAN SURFACE, AND OVER SAID SUBSTRATE FILM, A FILM OF A SOLUTION OF ALOWER TITANIUM ESTER HAVING ALKYL GROUPS CONTAINING UPON TO 12 CARBONATOMS, REMOVING THE SOLVENT FROM SAID SOLUTION, AND HYDROLYZING SAIDFILM IN AN ATMOSPHERE HAVING A RELATIVE HUMIDIETY OF BETWEEN ABOUT 20PERCENT AND 60 PERCENT THEREBY FORMING A FILM HAVING A THICKNESS NOTGREATER THAN 0.75 MICRON, REMOVING THE SUBSTRATE AND HYDROLYZED FILMFROM SAID SURFACE, SEPARATING THE SUBSTRATE FROM THE HYDROLYZED FILM,AND THEN FRACTURING SAID HYDROLYZED FILM, THEREBY FORMING ANACREPRODUCING PLATELETS.